1. Field of the Invention
This invention pertains to drilling fluids used in the drilling of oil and gas wells. In particular, the invention provides drilling fluid additives that provide significantly improved inhibition of cuttings dispersion and borehole stability.
2. Description of the Prior Art
In the drilling of an oil or gas well, a drilling fluid is circulated to the bottom of a borehole and ejected into the borehole from a drill bit. Usually, the drilling fluid returns to the surface of the well through the annulus of the borehole. Alternatively, the drilling fluid rises to the surface from the bottom of the borehole through a drill stem. Various additives may be added to the drilling fluid at the surface to enhance its performance.
In order to perform the boring operation most effectively, the drilling fluid should possess a number of properties. Thus, the drilling fluid should be capable of suspending the solid particles from the drilling operation when the drilling fluid ceases to circulate in the borehole, transporting the cuttings resulting from the drilling operation out of the borehole, avoiding excessive amounts of fluid loss during drilling operations, cooling and lubricating the drill bit, providing a wall cake upon the borehole wall, and providing hydrostatic pressure to the borehole to prevent the borehole from caving in or blowing out while using drilling fluids in a high pressure environment.
Water base drilling fluids typically include water and several additives or dispersants such as viscosifiers, thinners, dissolved salts, and solids. The term "water base" is used to distinguish these drilling fluids from oil base drilling fluids and oil-in-water emulsion drilling fluids and means that no oil is intentionally added to the drilling fluid. In drilling short distances, water alone has been used as the drilling fluid, but it lacks the lifting power, lubricity and borehole supporting properties provided by conventional deep well additives. More frequently, solids-containing water base fluids with greater density, lifting power and borehole supporting properties than water alone are employed as drilling fluids.
Many different additives and dispersants have been utilized that provide desired properties in drilling fluids. Exemplary are barium sulfate (barite), bentonite, low and medium yield clays, salt water clay, iron oxide, calcium carbonate, starch, carboxymethylcellulose, acrylonitrile, natural gum, molecularly dehydrated phosphate, tannin compound, quebracho, mined lignins, lignosulfonate, mica, sugar cane fibers, and granular materials. Bentonite is an especially common additive which gives the drilling fluid a thixotropic or gel structure; lignite is also a common additive which is added to the drilling fluid to control fluid loss; phosphate and lignosulfonate additives which deflocculate colloidal salts and thin the drilling fluid are also common.
Soft subterranean formations present the greatest problems for water base drilling fluids, especially formations of colloidal clays such as shales and gumbos, which tend to hydrate and swell when exposed to water base drilling fluids. Such swelling results in the formation of gummy, sludge-like paste in the borehole and reduces the cutting efficiency of the drill bit, thickens and impedes the circulation of drilling fluid, and plugs the drill. This is particularly true in drilling shales and related rock types where not only does the volume of the cuttings increase when hydrated, but the absorption water from the drilling fluid and dispersion of the shale increase both the viscosity of the circulated fluid and the torque demand on the drilling unit. Sometimes, enough torque can be exerted to cause the string to break or separate. Sometimes the viscosity increase results in fracturing the wellbore and lost circulation. Also, the shale and gumbo cuttings can stick to the surface of the drill bit and reduce its cutting efficiency by forming a layer between the bit and the formation being drilled. Further, the walls of the hole may collapse or become coated with hydrated cuttings, which makes it difficult to analyze the geologic structure penetrated, and the sludge of hydrated cuttings may stop the progress of a logging device or impede the operation thereof. Also, the collapse of the borehole wall may result in stuck pipe, poor circulation, poor cement and completion, and can result in excessive cost.
In spite of all of the above problems which are frequently encountered when softer formations are drilled with water base drilling fluids, such fluids are becoming increasingly attractive as compared to oil base drilling fluids. Generally, oil base drilling fluids are superior to water base drilling fluids by most measurements except one, environmental. Oil base drilling fluids are particularly difficult to dispose offshore, since such fluids cannot be dumped into the ocean, and it is in the offshore area that water base drilling fluids are drawing the most interest.
Calcium base drilling fluids, e.g., lime and gypsum, have been in use for quite some time, and are considered to work better than many other water base drilling fluids to reduce cuttings dispersion and increase wellbore stability, although still not nearly measuring up to the effectiveness of oil base drilling fluids. The mechanism of stabilization by the calcium cation is somewhat controversial, although there is some opinion that the calcium cation can exchange with the counterbalancing cations on clays in a formation and thus reduce the amount of hydration that can occur. Other features of the calcium base drilling fluids are also attractive, i.e., high tolerance to drill solids, reduced water dilution, decreased consumption of barite in weighted muds, and ability to control pH in spite of high CO.sub.2 influxes. Accordingly, it is readily apparent that the calcium base drilling fluids would be exceedingly attractive if they could approach the effectiveness of oil base drilling fluids in reducing cuttings dispersion and increasing wellbore stability.
Accordingly, the present invention is directed to overcoming the above noted problems in the art and in particular, problems of borehole stability and cuttings dispersion inhibition experienced in the use of calcium base muds.